My first questions are:
How large is the table top?
Are 3/4 inch (1x) boards thick enough for a table top?
Here are some ways to flatten the top:
* Pay a commercial service with a wide planer.
* Pay a commercial service with a drum sander.
* Hand planes and winding sticks. Requires much practice and skill.
* Router sled. Requires a decent router and building the sled. Some sanding after that. Search the internet.
* Belt sander and a good straightedge. Requires practice and skill to avoid making it worse, not better.
* Random orbital sander and a good straightedge. Check all directions very frequently. Don't sand in one place. If you have a low spot, you must bring the rest of the table down to match it. Takes time and patience, but low cost and relatively easy. Once flat, you can use the ROS with finer grits to do some finish sanding.
* Sanding block and sandpaper. When I was young, dumb, and poor, I hand-sanded a coffee table top with sandpaper wrapped around a block of cherry wood. Takes a lot of time and patience, but you earn bragging rights when you are done. Sand with the grain and check your work with a good straightedge.
Think about the thickness question.
Think about how you will prevent warping or cupping.
Think about how you will mount the top to the base in a way that allows for wood movement.